Method for operating a vehicle climate control system

ABSTRACT

A method for controlling torque of an air conditioner compressor is disclosed. In one example, the air conditioner compressor is a variable displacement compressor. The method may provide smooth transitions between different air conditioner compressor torques.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 13/243,607, entitled “METHOD FOR OPERATING A VEHICLE CLIMATE CONTROLSYSTEM,” filed on Sep. 23, 2011. The entire contents of theabove-referenced application are hereby incorporated by reference in itsentirety for all purposes.

FIELD

The present description relates to a method for operating a vehicleclimate control system. The method may be particularly useful formanaging starting and stopping of a climate control system.

BACKGROUND AND SUMMARY

Vehicle air conditioning systems can provide a driver with a comfortableenvironment during warm and/or humid ambient driving conditions. Airfrom the vehicle cabin is passed over an evaporator that cools the airand condenses water vapor from the air, thereby conditioning the cabinair to improve driver comfort. Air conditioning systems may be sizedwith a high cooling capacity so that the driver may be comfortableduring particularly warm days. However, it may not be desirable tooperate the air conditioner and air conditioner compressor all the timeonce a desired vehicle cabin temperature is reached.

Vehicle cabin temperature can be controlled for high capacity airconditioning systems via mechanically coupling and decoupling the airconditioner to the source supplying energy to the compressor. Forexample, the compressor clutch can be activated when cabin temperatureincreases above a desired cabin temperature by a predetermined amount.Conversely, the compressor clutch can be deactivated when cabintemperature decreases below the desired cabin temperature by apredetermined amount. However, mechanically coupling and decoupling theair conditioner compressor to the energy source can be noticeable andobjectionable to the driver of the vehicle.

The inventors herein have recognized the above-mentioned disadvantagesand have developed method for controlling an air conditioner compressorof a vehicle, comprising: reducing a refrigerant pressurization capacityof the air conditioning compressor before engaging and disengaging theair conditioner compressor to an energy conversion device that suppliesrotational energy to the air conditioner compressor.

By adjusting an air conditioner compressor displacement command beforeengaging and disengaging the air conditioner from an energy supply, itmay be possible to reduce torque disturbances of the vehicle driveline.For example, when a length of stroke of an air conditioner compressorpiston is reduced, an amount of torque to turn the compressor may bereduced. Consequently, changes in output torque of the energy source maybe less noticeable to the vehicle driver when the air conditionercompressor is coupled to an energy source while less torque is necessaryto turn the air conditioner compressor. Similarly, changes to the outputtorque of the energy source may be less noticeable to the driver whenthe air conditioner compressor is uncoupled from the energy source whileless torque is necessary to turn the air conditioner compressor.

The present description may provide several advantages. Specifically,the approach may improve transitions between loading and unloading anair conditioner compressor to a vehicle powertrain. In addition, theapproach may improve fuel control when the air conditioner compressor iscoupled to an engine since changes in engine load may be reduced.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 is a schematic diagram of a vehicle air conditioning system;

FIG. 2 is a schematic diagram of the energy conversion device of FIG. 1where the energy conversion device is an engine;

FIG. 3 is a block diagram of a control algorithm or method for operatingan air conditioning system of a vehicle;

FIG. 4 is an example plot of simulated signals of interest during airconditioning system operation;

FIG. 5 is an example plot of simulated signals of interest during airconditioner activation;

FIG. 6 is an example plot of simulated signals of interest during airconditioner deactivation;

FIG. 7 is an example simulated plot illustrating air conditioning systemcontrol temperature versus air conditioning system torque;

FIGS. 8A-8C are bar graphs illustrating examples of air conditioningtorque control;

FIG. 9 shows a method for controlling a vehicle air conditioning system;

FIG. 10 shows a method for adjusting an air conditioner displacementdemand; and

FIG. 11 shows a method for providing soft starting and stopping of avehicle air conditioner compressor.

DETAILED DESCRIPTION

The present description is related to controlling an air conditioningsystem of a vehicle. In one non-limiting example, the air conditioningsystem may be configured as illustrated in FIG. 1. Further, the airconditioning system may be coupled to an engine of a vehicle asillustrated in FIG. 2. In one example, the air conditioning system isoperated via a control system as illustrated in FIG. 3. FIGS. 4-6 showsignals of interest during air conditioning system operation. Airconditioning system temperature can be exchanged for air conditioningsystem torque as illustrated in FIG. 7. Air condition system torque canbe controlled as illustrated in FIG. 8C to improve vehicle operation.The methods of FIGS. 9-11 provide for controlling an air conditioningsystem with rapid response and smooth torque transitions betweendifferent operating modes.

Referring now to FIG. 1, air conditioning system 100 includes anevaporator 8 for cooling vehicle cabin air. Air is passed overevaporator 8 via fan 50 and directed around vehicle cabin 2. Climatecontroller 26 operates fan 50 according to operator settings as well asclimate sensors. Temperature sensor 4 provides an indication of thetemperature of evaporator 8 to climate controller 26. Cabin temperaturesensor 30 provides an indication of cabin temperature to climatecontroller 26. Similarly, humidity sensor 32 provides climate controller26 an indication of cabin humidity. Sun load sensor 34 provides anindication of cabin heating from sun light to climate controller 26.Climate controller also receives operator inputs from operator interface28 and supplies desired evaporator temperature and actual evaporatortemperature to energy conversion device controller 12.

Operator interface 28 allows an operator to select a desired cabintemperature, fan speed, and distribution path for conditioned cabin air.Operator interface 28 may include dials and push buttons to select airconditioning settings. In some examples, operator interface 28 mayaccept inputs via a touch sensitive display.

Refrigerant is supplied to evaporator 8 via evaporator valve 20 afterbeing pumped into condenser 16. Compressor 18 receives refrigerant gasfrom evaporator 8 and pressurizes the refrigerant. Heat is extractedfrom the pressurized refrigerant so that the refrigerant is liquefied atcondenser 16. The liquefied refrigerant expands after passing throughevaporator valve 20 causing the temperature of evaporator 8 to bereduced.

Compressor 18 includes a clutch 24, a variable displacement controlvalve 22, piston 80, and swash plate 82. Piston 80 pressurizesrefrigerant in air conditioning system which flows from air conditionercompressor 18 to condenser 16. Swash plate 82 adjusts the stroke ofpiston 80 to adjust the pressure at which refrigerant is output from airconditioner compressor 18 based on oil flow through variabledisplacement control valve 22. Clutch 24 may be selectively engaged anddisengaged to supply air conditioner compressor 18 with rotationalenergy from energy conversion device 10. In one example, energyconversion device 10 is an engine supplying rotational energy tocompressor 18 and wheels 60 via transmission 70. In other examples,energy conversion device 10 is an electrical motor supplying rotationalenergy to air conditioner compressor 18 and wheels 60 via transmission70. Rotational energy may be supplied to air conditioner compressor 18from energy conversion device 10 via belt 42. In one example, belt 42mechanically couples shaft 40 to air conditioner compressor 18 viaclutch 24. Shaft 40 may be an engine crankshaft, armature shaft, orother shaft.

In this way, the system of FIG. 1 provides rotational energy to an airconditioner compressor to cool the cabin of a vehicle. Specifically, theair conditioner compressor provides a negative torque to load the energyconversion device and compress the refrigerant so that the refrigerantcan be subsequently expanded in order to cool the vehicle cabin. Theamount of negative torque provided to the energy conversion device bythe air conditioner compressor can be adjusting via the clutch and anactuator or valve that adjusts the variable displacement pump.

Referring to FIG. 2, one example of an energy conversion device isshown. In particular, energy conversion device 10 is an internalcombustion engine, comprising a plurality of cylinders, one cylinder ofwhich is shown in FIG. 1, is controlled by electronic energy conversiondevice controller 12. Engine 10 includes combustion chamber 230 andcylinder walls 232 with piston 236 positioned therein and connected toshaft 40 which is a crankshaft. Combustion chamber 230 is showncommunicating with intake manifold 244 and exhaust manifold 248 viarespective intake valve 252 and exhaust valve 254. Each intake andexhaust valve may be operated by an intake cam 251 and an exhaust cam253. Alternatively, one or more of the intake and exhaust valves may beoperated by an electromechanically controlled valve coil and armatureassembly. The position of intake cam 251 may be determined by intake camsensor 255. The position of exhaust cam 253 may be determined by exhaustcam sensor 257.

Fuel injector 266 is shown positioned to inject fuel directly intocylinder 230, which is known to those skilled in the art as directinjection. Alternatively, fuel may be injected to an intake port, whichis known to those skilled in the art as port injection. Fuel injector266 delivers liquid fuel in proportion to the pulse width of signal FPWfrom energy conversion device controller 12. Fuel is delivered to fuelinjector 266 by a fuel system (not shown) including a fuel tank, fuelpump, and fuel rail (not shown). Fuel injector 266 is supplied operatingcurrent from driver 268 which responds to energy conversion devicecontroller 12. In addition, intake manifold 244 is shown communicatingwith optional electronic throttle 262 which adjusts a position ofthrottle plate 264 to control air flow from air intake 242 to intakemanifold 244. In one example, a low pressure direct injection system maybe used, where fuel pressure can be raised to approximately 20-30 bar.Alternatively, a high pressure, dual stage, fuel system may be used togenerate higher fuel pressures.

Distributorless ignition system 288 provides an ignition spark tocombustion chamber 230 via spark plug 292 in response to energyconversion device controller 12. Universal Exhaust Gas Oxygen (UEGO)sensor 226 is shown coupled to exhaust manifold 248 upstream ofcatalytic converter 270. Alternatively, a two-state exhaust gas oxygensensor may be substituted for UEGO sensor 226.

Converter 270 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 270 can be a three-way type catalyst inone example.

Energy conversion device controller 12 is shown in FIG. 1 as aconventional microcomputer including: microprocessor unit 202,input/output ports 204, read-only memory 206, random access memory 208,keep alive memory 210, and a conventional data bus. Energy conversiondevice controller 12 is shown receiving various signals from sensorscoupled to energy conversion device 10, in addition to those signalspreviously discussed, including: engine coolant temperature (ECT) fromtemperature sensor 212 coupled to cooling sleeve 214; a position sensor284 coupled to an accelerator pedal 280 for sensing force applied byfoot 282; a measurement of engine manifold pressure (MAP) from pressuresensor 222 coupled to intake manifold 244; an engine position sensorfrom a Hall effect sensor 218 sensing position of shaft 40; ameasurement of air mass entering the engine from sensor 220; and ameasurement of throttle position from sensor 258. Barometric pressuremay also be sensed (sensor not shown) for processing by energyconversion device controller 12. In a preferred aspect of the presentdescription, engine position sensor 218 produces a predetermined numberof equally spaced pulses every revolution of the crankshaft from whichengine speed (RPM) can be determined.

In some embodiments, the engine may be coupled to an electricmotor/battery system in a hybrid vehicle. The hybrid vehicle may have aparallel configuration, series configuration, or variation orcombinations thereof. Further, in some embodiments, other engineconfigurations may be employed, for example a diesel engine.

During operation, each cylinder within the engine typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 254 closes and intake valve 252 opens. Airis introduced into combustion chamber 230 via intake manifold 244, andpiston 236 moves to the bottom of the cylinder so as to increase thevolume within combustion chamber 230. The position at which piston 236is near the bottom of the cylinder and at the end of its stroke (e.g.when combustion chamber 230 is at its largest volume) is typicallyreferred to by those of skill in the art as bottom dead center (BDC).During the compression stroke, intake valve 252 and exhaust valve 254are closed. Piston 236 moves toward the cylinder head so as to compressthe air within combustion chamber 230. The point at which piston 236 isat the end of its stroke and closest to the cylinder head (e.g. whencombustion chamber 230 is at its smallest volume) is typically referredto by those of skill in the art as top dead center (TDC). In a processhereinafter referred to as injection, fuel is introduced into thecombustion chamber. In a process hereinafter referred to as ignition,the injected fuel is ignited by known ignition means such as spark plug292, resulting in combustion. During the expansion stroke, the expandinggases push piston 236 back to BDC. Shaft 40 converts piston movementinto a rotational torque of the rotary shaft. Finally, during theexhaust stroke, the exhaust valve 254 opens to release the combustedair-fuel mixture to exhaust manifold 248 and the piston returns to TDC.Note that the above is shown merely as an example, and that intake andexhaust valve opening and/or closing timings may vary, such as toprovide positive or negative valve overlap, late intake valve closing,or various other examples.

FIG. 3 is a block diagram of a controller for operating an airconditioning system of a vehicle. The controller may be executed viainstructions in electronic controller 12 operating in the systemsdescribed by FIGS. 1 and 2. Controller 300 includes a first section302-332 and 350 for adjusting displacement of a variable displacementair conditioner compressor (e.g., compressor 18 of FIG. 1). Controller300 includes a second section 340-344 for adjusting air conditionercompressor clutch state which allows rotational energy to be selectivelytransferred to an air conditioner compressor.

At 302, desired evaporator temperature is input to controller 300. Inone example, the desired evaporator temperature may be requested from aclimate control module in response to operator inputs and airconditioning system inputs. Further, the desired evaporator temperaturecan converge to an evaporator control temperature when the airconditioner compressor clutch is activated. The desired evaporatortemperature can converge to ambient temperature when the air conditionercompressor clutch is not activated. The desired evaporator temperatureis directed to 304 and 350.

At 350, a feed forward gain is applied to the desired evaporatortemperature. In one example, the gain is empirically determined andstored in memory. For example, if the desired evaporator temperature is20° C., a displacement valve command of 60% duty cycle may be extractedfrom a table that is indexed via the 20° C. desired evaporatortemperature. The 60% duty cycle may then be directed to the displacementvalve. In one example, the feed forward gain may be indexed via one ormore of the variables of fan speed, desired evaporator temperature,ambient temperature, and solar load. The feed forward gain is directedfrom 350 to 326.

At 304, controller 300 determines an expected evaporator temperature inresponse to the desired evaporator temperature, sensed evaporatortemperature, and air conditioner compressor clutch state. In oneexample, the expected evaporator temperature is determined according tothe state of the air conditioner compressor clutch.

During air conditioner compressor clutch off conditions, the expectedevaporator temperature (exp_evp_tmp) is given by exp_evp_tmp(K)=filt_dsd_evp_tmp (K−n). Where K is an integer representing theK^(th) determination of exp_evp_tmp, n is an integer representing adelay time between the present determination of exp_evp_tmp andfilt_dsd_evp_tmp, and where filt_dsd_evp_tmp is the filtered desiredevaporator temperature. The delay n may be empirically determined andstored in controller memory via commanding the air conditionercompressor clutch to an off state from a clutch on state and recordingan amount time before the evaporator reaches a final temperature that isbased on ambient conditions. The amount of time to reach ambienttemperature is a function of the evaporator volume, fan speed, initialevaporator temperature, and ambient conditions. Thus, exp_evp_tmp (K)takes on the value of filt_dsd_evp_tmp delayed by n execution cycles ofcontroller 300.

In one example, filt_dsd_evp_tmp is determined from the desiredevaporator temperature and first order filter expressed asfilt_dsd_evp_tmp (K)=filt_dsd_evp_tmp (K−1)+(1−τ_(off))·(dsd_evp_tmp(K)−filt_dsd_evp_tmp (K−1)). Where dsd_evp_tmp is desired evaporatortemperature from 302, K is an integer representing the K^(th)determination of filt_dsd_evp_tmp, τ_(off) is related to an airconditioner compressor off filter time constant and the sampling rate ofdesired evaporator temperature. The air conditioner compressor offfilter time constant represents a rise rate of desired evaporatortemperature from the present evaporator temperature to the finalevaporator temperature. The final evaporator temperature may beestimated as ambient temperature while the present evaporatortemperature may be sensed at 308. The air conditioner compressor offfilter time constant may be empirically determined and stored in a tableor function that is indexed via the present evaporator temperature andthe final evaporator temperature.

In this way, desired evaporator temperature is filtered and delayedbefore being used to determine an error between the desired evaporatortemperature and the actual evaporator temperature. By converting thedesired evaporator temperature into an expected evaporator temperature,the feedback section (e.g., 306-332) of controller 300 can compensatefor the delay time and the time constant of the physical system shown inFIGS. 1 and 2 via allowing the actual evaporator temperature to reachthe desired evaporator temperature before taking additional controlactions beyond feed forward gain operating on the desired evaporatortemperature. Further, an observed output of the air conditioning systemillustrated in FIGS. 1 and 2 does not have to be modeled and directedthrough a delay to compare with the actual evaporator temperature aswould be done with a Smith Predictor. Instead, the control system inputdesired evaporator temperature is modified before being used to providean evaporator temperature error signal or value.

During air conditioner compressor clutch on conditions, the expectedevaporator temperature (exp_evp_tmp) is given by exp_evp_tmp(K)=filt_dsd_evp_tmp (K−m). Where K is an integer representing theK^(th) determination of exp_evp_tmp, m is an integer representing adelay between the present determination of exp_evp_tmp andfilt_dsd_evp_tmp, and where filt_dsd_evp_tmp is the filtered desiredevaporator temperature. The delay m may be empirically determined andstored in controller memory via commanding the air conditionercompressor clutch to an on state and recording an amount time before theevaporator reaches a final drawdown temperature that is based on the airconditioner compressor pump displacement and ambient conditions. Thus,exp_evp_tmp (K) takes on the value of filt_dsd_evp_tmp delayed by mexecution cycles of controller 300.

In one example, filt_dsd_evp_tmp is determined from the desiredevaporator temperature and first order filter expressed asfilt_dsd_evp_tmp (K)=filt_dsd_evp_tmp (K−1)+(1−τ_(on))·(dsd_evp_tmp(K)−filt_dsd_evp_tmp (K−1)). Where dsd_evp_tmp is desired evaporatortemperature from 302, K is an integer representing the K^(th)determination of filt_dsd_evp_tmp, τ_(on) is related to an airconditioner compressor on filter time constant and the sampling rate ofdesired evaporator temperature. The air conditioner compressor on filtertime constant represents a drop rate of desired evaporator temperaturefrom the present evaporator temperature to the desired evaporatortemperature. The air conditioner compressor on filter time constant maybe empirically determined and stored in a table or function that isindexed via the present evaporator temperature and the desiredevaporator temperature.

Thus, expected evaporator temperature can be determined and updatedwhether the air conditioner compressor clutch is engaged or disengaged.As such, the feedback section (e.g., 306-332) of controller 300 operatesbased on the expected evaporator temperature rather than a differencebetween a desired evaporator temperature and an actual evaporatortemperature. By modifying the desired evaporator temperature to providean expected evaporator temperature, controller 300 can mitigate thepossibility of over correcting or driving the air conditioner compressordisplacement command.

At 308, evaporator temperature is sensed or estimated. In one example,evaporator temperature is sensed via a thermocouple or a thermister asindicated in FIG. 1. The sensed evaporator temperature is passed on to304, 306, and 342.

At 306, actual evaporator temperature is subtracted from expectedevaporator temperature to provide an evaporator temperature error. Theevaporator temperature error is a basis for feedback adjustments to thedisplacement of the variable displacement air conditioner compressor.Evaporator temperature error is directed to 310.

At 310, controller 300 judges whether or not to evaporator temperatureerror is greater than a threshold level. If so, controller 300 proceedsto 318. Otherwise, controller 300 proceeds to 310. Thus, if theevaporator temperature error is less than the threshold, a PIDcontroller operates on the evaporator temperature error. Otherwise, ahigh gain no memory gain operates on the evaporator temperature error.

In an alternative example, controller 300 may supply evaporatortemperature error to both paths comprising 318, 312, 314, and 316 duringselected operating conditions. For example, if evaporator temperatureerror is less than 5° C. but more than 3° C., 318, 312, 314, and 316 mayreceive the evaporator temperature error value. However, if evaporatortemperature error is greater than 5° C., only 318 receives theevaporator temperature error value. Further, if evaporator temperatureerror is less than 3° C., only 312, 314, and 316 receive the evaporatortemperature error value.

At 318, the evaporator temperature error is multiplied by a high gain.The high gain may be linear, quadratic, or of a higher order.Alternatively, the high gain may be a single value for negativeevaporator temperature errors or a single value for positive evaporatortemperature errors. In one example, high gain determined from a functionor table stored in memory that is indexed via evaporator temperatureerror. For example, if the evaporator temperature error is 10° C., atable is indexed using 10° C. and the variable displacement controlvalve command is adjusted by 15%. The high gain at 318 contains nomemory so that only the present evaporator temperature error is operatedon by controller 300 and not past errors in evaporator temperature. Thegain output from 318 is supplied to 322.

At 322, controller 300 limits rate increases in the variabledisplacement control valve command. Further, in some examples controller300 applies a low pass filter to adjustments for the variabledisplacement control valve. For example, if a change in the variabledisplacement control valve is greater than 40%, the change in thevariable displacement control command is held at 40%. Further, thechange may be filtered to smooth adjustments to the variabledisplacement command. The limited variable displacement control valvecommand is directed to summing junction 326.

At 312, controller 300 proportionally adjusts the evaporator temperatureerror signal by multiplying it by a proportional amount to provide aproportional air conditioner compressor displacement command adjustmentterm. For example, if the evaporator temperature error is 10° C., it maybe multiplied by 0.5 to provide a value of 5. The proportionatelyadjusted evaporator temperature error is directed to summing junction324.

At 314, controller 300 integrates the evaporator temperature error andthen multiplies the integrated temperature error by a predeterminedvalue to provide an integral air conditioner compressor displacementcommand adjustment term. In one example, the evaporator temperatureerror may be integrated via a trapezoidal method of integration. Thus,the integrated amount includes present and past values of evaporatortemperature error and is therefore considered to have memory of pastevaporator temperature error. The integrated evaporator temperatureerror is directed to windup limiter 320.

At 320, controller 300 limits the maximum value of the integratedevaporator temperature error so that if the evaporator temperature errorchanges sign, controller 300 may respond quickly without having to cleara large integrated value of evaporator temperature. Output from winduplimiter 320 is directed to summing junction 324.

At 316, controller 300 takes a derivative of evaporator temperatureerror and multiplies it by a predetermined value to provide a derivativeair conditioner compressor displacement command adjustment term. In oneexample, the derivative is determined from a change between a presentand a most recent past evaporator temperature error as well as the timebetween samples. For example, the derivative term may be determined asevap_deriv=(evap_tmp_error (K−1)−evap_tmp_error(K))/D where K is thepresent sample number and D is the number of seconds per sample. Thederivative term is directed to summing junction 324.

At 324, the proportional, derivative, and integral terms are summed toprovide an output to the PID portion of controller 300. Thus, when theevaporator error is less than a threshold level, the evaporator error isoperated on via a PID controller. However, in some examples, if theevaporator error is greater than a threshold level, the output of summer324 is forced to zero. The output from summer 324 is directed to summer326.

At 326, the output of the feed forward gain at 350 is added to theoutput of the PID controller as summed at 324 and the output of highgain limiter 322. Thus, during some conditions, controller 300 providesa control signal that is based on feed forward gain and high gaincontrol without using memory in the control output. During otherconditions, controller 300 provides a control signal that is based onfeed forward gain and a PID section that includes using memory todetermine the control output. Thus, controller 300 includes memory basedoutput and memory-less output. Controller 300 directs the output ofsummer 326 to 328.

At 328, controller 300 provides a soft start and stop feature that actsto reduce disturbances to the energy conversion device supplyingrotational energy to the air conditioner compressor. In particular, whencontroller 300 judges to activate the air conditioner compressor clutchto couple the air conditioner compressor to the energy conversiondevice, the air conditioner compressor displacement valve is commandedto a reduced or minimum displacement before the air conditionercompressor clutch is engaged. The air conditioner compressordisplacement valve is commanded to an increased value a predeterminedamount of time after the air conditioner compressor clutch has beenengaged. In one example, the air conditioner compressor displacement isgradually adjusted, such as by filtering the command, or ramped, to aduty cycle as output from 326 after a predetermined amount of time sincethe air conditioner compressor clutch is engaged.

Conversely, when controller 300 judges to deactivate the air conditionercompressor clutch to uncouple the air conditioner compressor to theenergy conversion device, the air conditioner compressor displacementvalve is commanded to a reduced or minimum displacement before the airconditioner compressor clutch is disengaged. The air conditionercompressor displacement valve is commanded to a decreased value apredetermined and the air conditioner compressor clutch is disengaged apredetermined amount of time after the air conditioner compressordisplacement is degreased to a lower or minimum displacement. In oneexample, the air conditioner compressor displacement is ramped as soonas it is decided to disengage the air conditioner compressor clutch. Thesoft start or stop adjusted air conditioner compressor displacementcontrol value is routed from 328 to 330.

At 330, controller 300 controls air conditioner torque in response topowertrain torque requirements and available energy conversion devicetorque as described in greater detail with regard to FIGS. 9-10. Duringsome vehicle operating conditions, it may be desirable to reduce thenegative or resistive torque that the air conditioner compressor appliesto the energy conversion device so that additional torque may besupplied by the energy conversion device to propel the vehicle orincrease the output of other vehicle systems. For example, during acondition where an operator substantially depresses an acceleratorpedal, it may be desirable to reduce the amount of energy conversiondevice torque consumed by the air conditioner compressor. In anotherexample, a load of an alternator may increase to a level where it isdesirable to reduce the torque supplied to the air conditionercompressor to increase alternator output. In still another example, itmay be desirable to reduce torque supplied to an air conditionercompressor during engine idle conditions where MAP pressure is greaterthan a threshold so that MAP may be reduced to increase brake boostervacuum. Thus, there are many conditions where it may be desirable toreduce torque supplied to an air conditioner compressor.

One way to reduce torque supplied to an air conditioner compressor is toopen the air conditioner clutch. FIG. 8B described below in greaterdetail describes how driveline torque can be increased when there is anincreasing request for additional driveline torque from the energyconversion device.

In another example, as described in greater detail with regard to FIGS.8C and 10, air conditioner negative torque applied to the energyconversion device can be reduced in response to an amount of requesteddriveline torque increase. By reducing air conditioner compressornegative torque in proportion to an increase in requested drivelinetorque, air conditioning cooling capacity can be reduced so that therequested drive line torque may be provided. Similarly, compressortorque may be adjusted responsive to other energy conversion devicetorque requests. For example, compressor torque may be reduced inresponse to a power take off device for operating a hydraulic pump,and/or an alternator load, and/or a power steering torque demand, and/ora request for additional engine vacuum. Controller 300 supplies anadjusted air conditioner compressor displacement command to 332 afterair conditioner compressor torque may be limited responsive to othertorque demands on the energy conversion device.

At 332, controller 300 adjusts stroke of the air conditioner compressorpiston via the displacement command to change the pressure output of theair conditioner compressor. In one example, air conditioner displacementcommand is adjusted via varying a duty cycle of a waveform controlling avalve (e.g., 20 of FIG. 1) that regulates fluid flow to control airconditioner compressor piston stroke. In other examples, an electricalmotor or solenoid may be supplied a varying voltage so as to control airconditioner compressor pressure capacity.

In this way, controller 300 adjusts the variable displacement airconditioner compressor to provide varying levels of air conditionerevaporator cooling capacity while at the same time controlling thetorque that the air conditioner compressor applies to an energyconversion device. In particular, the evaporator cooling capacity may beincreased by increasing the stroke of the air conditioner compressorpiston, thereby increasing the pressure output of the air conditionercompressor.

At 340, controller 300 receives operator and system inputs forcontrolling a clutch of an air conditioner compressor that selectivelyallows rotational energy to be supplied from an energy conversion deviceto the air conditioner compressor. In one example, the operator andsystem inputs include but are not limited to solar load, fan speedcommand, cabin temperature demand, evaporator temperature, humiditysensor, climate control mode (e.g., cool; heat; defrost). Operator andsystem inputs are passed from 340 to 342.

At 342, controller 300 applies logic to determine whether or not toactuate an air conditioner clutch so that the air conditioner compressorcan pressurize refrigerant to reduce the temperature of an evaporator(e.g., evaporator 8 of FIG. 1). For example, if desired vehicle cabintemperature is greater than actual cabin temperature, the airconditioner compressor clutch can be activated to allow rotationalenergy to be transferred from an energy conversion device to the airconditioner compressor so that evaporator temperature may be lowered,thereby reducing vehicle cabin temperature. Further, when the vehiclecabin temperature is cooled to a level that is less than the desiredvehicle cabin temperature, the air conditioner compressor clutch can bedeactivated to stop rotational energy from being transferred from theenergy conversion device to the air conditioner compressor. Controller300 adjusts air conditioning clutch state by directing signals to 344.

At 344, controller 300 adjusts the state of an air conditionercompressor clutch. In one example, the air conditioner compressor clutchis electromechanically actuated. In another example, the air conditionercompressor clutch may be hydraulically actuated. Thus, electricalcurrent or hydraulic fluid may be used to activate or deactivate the airconditioner clutch. Further, the air conditioner compressor clutchcommand state output at 344 may be delayed from the desired airconditioner compressor clutch state determined at 342 in order tofacilitate a soft start/stop of the air condition system and airconditioner compressor. The amount of delay time may be constant or varywith air conditioning system operating conditions as described withregard to FIGS. 5 and 6.

Referring now to FIG. 4, an example plot of simulated signals ofinterest during air conditioning system operation is shown. The signalsof FIG. 4 may be provided via controller 12 of FIGS. 1 and 2 executinginstructions of the controller described in FIG. 3.

FIG. 4 includes two plots. The first plot from the top of FIG. 4 is aplot of evaporator temperature versus time. The X axis represents timeand time increases from left to right. The Y axis represents evaporatortemperature and evaporator temperature increases in the direction of theY axis arrow. Curve 402 represents desired air conditioner evaporatortemperature (e.g., 302 of FIG. 3). Curve 404 represents expected airconditioner evaporator temperature (e.g., 304 of FIG. 3). Line 410represents an air conditioner evaporator control temperature level(e.g., a temperature the air conditioning evaporator is controlled towhen the air conditioner compressor clutch is activated).

The second plot from the top of FIG. 4 is a plot of air conditionercompressor clutch state. The X axis represents time and time increasesfrom left to right. The Y axis represents air conditioner compressorclutch state and air conditioner compressor clutch state is open nearthe X axis (e.g., a low level) and air conditioner compressor clutchstate is closed near the Y axis arrow (e.g., high level).

At time T₀, the air conditioner compressor clutch is activated and thedesired evaporator temperature 402 (e.g., 302 of FIG. 3) as provided bya climate controller (e.g., 26 of FIG. 1) and the expected evaporatortemperature 404 (e.g., 304 of FIG. 3) are near the air conditionerevaporator control temperature level 410.

At time T₁, the air conditioner compressor clutch is cycled off so thatrotational energy from an energy conversion device is not transferred tothe air conditioner compressor. The air conditioner compressor clutchmay be cycled off to conserve energy, in response to a request from thevehicle operator, or in response to other air conditioning system input.

At time T₂, the desired air conditioner evaporator temperature 402begins to move away from the air conditioner evaporator controltemperature 410 since the air conditioner compressor clutch isdisengaged.

At time T₃, the expected air conditioner evaporator temperature 404begins to increase. The rate of expected air conditioner evaporatortemperature increase can be less than or equal to the rate of desiredair conditioner evaporator temperature increase depending on the filtertime constant selected at 304 of FIG. 3. Similarly, the rate of expectedair conditioner evaporator temperature decrease can be less than orequal to the rate of desired air conditioner evaporator temperaturedecrease depending on the filter time constant selected at 304 of FIG.3. The time between T₁ and T₂ represents the time delay selected at 304of FIG. 3.

At time T₄, the air conditioner compressor clutch is reactivated so thatrotational energy is transferred from the energy conversion device tothe air conditioner compressor. Shortly thereafter, the desiredevaporator temperature 402 and the expected evaporator temperature 404begin to be reduced.

Thus, the expected evaporator temperature 404 is delayed from thedesired evaporator temperature so that the evaporator temperature error(e.g., 306 of FIG. 3) is more closely related to the actual evaporatortemperature. Consequently, oscillations in controller output and actualtemperature may be reduced even though there may a significant phasedelay from the desired evaporator temperature and the actual evaporatortemperature. Further, the magnitude of the desired evaporatortemperature may be reduced during dynamic conditions so as to reduce thepossibility of over driving the air conditioner compressor displacementcommand (e.g., 332 of FIG. 3) during dynamic conditions.

Referring now to FIG. 5, an example plot of simulate signals of interestduring air conditioner activation is shown. In particular, an examplesoft start of an air conditioning system is shown. The signals of FIG. 5may be provided via controller 12 of FIGS. 1 and 2 executinginstructions of the controller described in FIG. 3.

FIG. 5 includes three plots. The first plot from the top of FIG. 5 is aplot of an air conditioner compressor piston displacement or strokecontrol signal (e.g., 328 of FIG. 3) versus time. The pumping pressurecapacity of the air conditioner compressor increases as the pistondisplacement command increases. The X axis represents time and timeincreases from left to right. The Y axis represents compressordisplacement command and the air conditioner compressor displacementcommand increases in the direction of the Y axis arrow, therebyincreasing compressor pressure capacity.

The second plot from the top of FIG. 5 is a plot of desired airconditioner compressor clutch state. The X axis represents time and timeincreases from left to right. The Y axis represents desired airconditioner compressor clutch state and desired compressor clutch stateis open near the X axis (e.g., a low level) and desired compressorclutch state is closed near the Y axis arrow (e.g., high level). Thedesired air conditioner clutch state may be determined according to airconditioner inputs as described at 342 of FIG. 3.

The third plot from the top of FIG. 5 is a plot of air conditionercompressor clutch command state. The X axis represents time and timeincreases from left to right. The Y axis represents air conditionercompressor clutch command state and compressor clutch command stateopens the air conditioner compressor clutch near the X axis (e.g., a lowlevel) and compressor clutch command state closes the air conditionercompressor clutch near the Y axis arrow (e.g., high level). The desiredair conditioner clutch state may be determined according to airconditioner inputs as described at 342 of FIG. 3.

At time T₀, the desired air conditioner compressor clutch state is at alow level indicating that the air conditioner clutch is not to beactivated. The air conditioner compressor clutch command state is alsoat a low level indicating that the air conditioner clutch is notactivated. Further, the air conditioner compressor displacement commandsignal is also at a low level, thereby reducing the air conditionercompressor pressure capacity and the amount of torque applied to theenergy conversion device.

At time T₁, the desired air conditioner compressor clutch state iscycled on so that rotational energy from an energy conversion device canbe transferred to the air conditioner compressor. The air conditionercompressor clutch may be cycled on to start the air conditioningreducing vehicle cabin temperature or after the air conditionercompressor has been cycled off based on air conditioning system inputs.The air conditioner compressor clutch command state remains at a lowlevel indicating that the air conditioner clutch is not immediatelyactivated when the desired air conditioner compressor clutch state ischanged. Further, the air conditioner compressor displacement commandremains at a low or minimum level so that compressor pressure capacityis at a low or minimum level.

At time T₂, the desired air conditioner compressor clutch state remainsin a state to engage the air conditioner compressor clutch. The airconditioner compressor clutch command state transitions to a high levelindicating that the air conditioning clutch is commanded to an activeengaged state where rotational energy from the energy conversion deviceis transferred to the air conditioner compressor. However, the airconditioner compressor displacement control signal remains at a lower orminimum level so that when the air conditioner compressor clutch isengaged, a low level torque is applied to the energy conversion device.Thus, during air conditioner compressor clutch engagement, only a smallload is applied to the energy conversion device from the air conditionercompressor.

At time T₃, the desired air conditioner compressor clutch state and theair conditioner compressor clutch command state are held at higherlevels. Further, the air conditioner compressor displacement commandsignal begins to be ramped from the low or minimum level to a level thatprovides the desired evaporator temperature.

The time between T₂ and T₃ may be a constant or it may be adjusted inresponse to air conditioning system or energy conversion deviceoperating conditions. For example, the time from T₂ to T₃ may be a firstamount of time when engine speed is a first engine speed, and the timefrom T₂ to T₃ may be a second amount of time, the second amount of timeshorter than the first amount of time, when engine speed is a secondengine speed, the second engine speed greater than the first enginespeed. Further, the time may be increased as the difference between theinitial and final evaporator temperatures increases. In other words, theramp rate between initial evaporator temperature and final evaporatortemperature can be adjusted according to energy conversion deviceconditions, air conditioning system conditions, and vehicle conditions.

For example, when the energy conversion device coupled to the airconditioner compressor is an engine, the air conditioner compressordisplacement command signal may ramp up at a first rate when the airconditioner compressor is activated at engine idle speed and load (e.g.,800 RPM and 0.12 load). On the other hand, when the engine is operatingat a higher speed and load (e.g., 2000 RPM and 0.3 load), the airconditioner compressor displacement control signal may ramp up at asecond rate, the second rate greater than the first rate. The airconditioner compressor displacement control signal may ramp up at ahigher rate during conditions where the faster ramp rate is less likelyto be noticed by the operator. Further, the ramp up rate may beincreased during conditions where the energy conversion device can reactfaster to counteract the additional compressor torque. For example, asmentioned above, a compressor displacement command may be ramped upfaster so as to increase compressor pressure capacity when an engine isoperated at speeds above idle speed since higher engine speed providesadditional combustion events, thereby reducing the time it takes betweenand engine control adjustment and torque to counteract the airconditioner compressor torque.

A different compressor displacement ramping up strategy may be providedwhen the air conditioner compressor is coupled to an electric motor. Forexample, the air conditioner compressor displacement command ramp uprate may be increased at a first rate when the motor speed is less thanbase speed (e.g., motor speed where full motor torque is available andabove which less than full motor torque is available). However, if theelectric motor is at a speed greater than base speed, the airconditioner compressor displacement command ramp up rate may be reducedas compared to the first rate to account for less available motor torqueat higher motor speeds.

In addition, the air conditioner compressor displacement ramp up ratemay be adjusted depending on air conditioner system operatingconditions. For example, the ramp up rate may be increased when thedifference between an initial evaporator temperature and a final ordesired evaporator temperature is less than a first threshold (e.g., 10°C.). The ramp up rate may be decreased when the difference between theinitial evaporator temperature and the desired evaporator temperature isgreater than a second threshold (e.g., 15° C.).

The time between T₂ and T₃ allows the torque transfer amount from theengine to the air conditioner compressor to stabilize before the airconditioner compressor displacement is adjusted. The amount of timebetween T₂ and T₃ can be adjusted for air conditioner system operatingconditions, vehicle operating conditions, and energy conversion deviceconditions. For example, the time between T₂ and T₃ can be increased ifthe energy conversion device has been operating for less than apredetermined amount of time. In an alternative example, the timebetween T₂ and T₃ can be increased when the energy conversion device isan engine operating with fewer than its total number of cylinders. Inparticular, when the engine is operating with deactivated cylinders thetime between T₂ and T₃ can be increased as compared to when the engineis operating with a greater number or all engine cylinders.

In this way, the possibility of large increases in torque between theenergy conversion device and the air conditioner compressor may bereduced. As a result, activation of the air conditioner systemactivation may be less noticeable to a driver.

Referring now to FIG. 6, an example plot of simulated signals ofinterest during air conditioner deactivation is shown. In particular, anexample soft stop of an air conditioning system is shown. The signals ofFIG. 6 may be provided via controller 12 of FIGS. 1 and 2 executinginstructions of the controller described in FIG. 3.

FIG. 6 includes three plots. The first plot from the top of FIG. 6 is aplot of a compressor piston displacement or stroke control signal (e.g.,328 of FIG. 3) versus time. The pumping pressure capacity of the airconditioner compressor increases as the piston displacement commandincreases. The X axis represents time and time increases from left toright. The Y axis represents compressor displacement command and the airconditioner compressor displacement command increases in the directionof the Y axis arrow, thereby increasing compressor pressure capacity.

The second plot from the top of FIG. 6 is a plot of desired airconditioner compressor clutch state. The X axis represents time and timeincreases from left to right. The Y axis represents desired airconditioner compressor clutch state and desired compressor clutch stateis open near the X axis (e.g., a low level) and desired compressorclutch state is closed near the Y axis arrow (e.g., high level). Thedesired air conditioner clutch state may be determined according to airconditioner inputs as described at 342 of FIG. 3.

The third plot from the top of FIG. 6 is a plot of air conditionercompressor clutch command state. The X axis represents time and timeincreases from left to right. The Y axis represents air conditionercompressor clutch command state and compressor clutch command stateopens the air conditioner compressor clutch near the X axis (e.g., a lowlevel) and compressor clutch command state closes the air conditionercompressor clutch near the Y axis arrow (e.g., high level). The desiredair conditioner clutch state may be determined according to airconditioner inputs as described at 342 of FIG. 3.

At time T₀, the desired air conditioner compressor clutch state is at ahigher level indicating that the air conditioner clutch is activated.The air conditioner compressor clutch command state is also at a higherlevel indicating that the air conditioner clutch is activated. Further,the air conditioner compressor displacement command signal is also at ahigher level, thereby increasing the air conditioner compressor pressurecapacity and the amount of torque applied to the energy conversiondevice.

At time T₁, the desired air conditioner compressor clutch state iscycled off so that rotational energy from an energy conversion devicecannot be transferred to the air conditioner compressor, but the airconditioner compressor clutch remains engaged at T₁. The air conditionercompressor clutch may be cycled off to stop the air conditioning so thatvehicle cabin temperature can be increased or to reduce energyconsumption. The air conditioner compressor clutch command state remainsat a higher level indicating that the air conditioner clutch is notimmediately deactivated when the desired air conditioner compressorclutch state is changed. Further, the air conditioner compressordisplacement command remains at a higher level so that compressorpressure capacity remains higher.

At time T₂, the desired air conditioner compressor clutch state is heldat lower level. However, the air conditioner compressor clutch commandstate is held at a higher level so that the air conditioner compressorclutch remains engaged to allow torque transfer from the electric energyconversion device to the air conditioner compressor. In addition, theair conditioner compressor displacement command signal begins to beramped from the higher level to a lower or minimum level that providesreduced compressor output pressure and less compressor torque.

The time between T₁ and T₂ may be a constant or it may be adjusted inresponse to air conditioning system or energy conversion deviceoperating conditions. For example, the time from T₁ to T₂ may be a firstamount of time when engine speed is a first engine speed, and the timefrom T₁ to T₂ may be a second amount of time, the second amount of timeshorter than the first amount of time, when engine speed is a secondengine speed, the second engine speed greater than the first enginespeed. Further, the time may be increased as the difference between theinitial and final evaporator temperatures increases. In other words, theramp rate between initial evaporator temperature and final evaporatortemperature can be adjusted according to energy conversion deviceconditions, air conditioning system conditions, and vehicle conditions.

For example, when the energy conversion device coupled to the airconditioner compressor is an engine, the air conditioner compressordisplacement command signal may ramp down at a first rate when the airconditioner compressor is deactivated at engine idle speed and load(e.g., 800 RPM and 0.12 load). On the other hand, when the engine isoperating at a higher speed and load (e.g., 2000 RPM and 0.3 load), theair conditioner compressor displacement control signal may ramp down ata second rate, the second rate greater than the first rate. The airconditioner compressor displacement control signal may ramp down at ahigher rate during conditions where the faster ramp down rate is lesslikely to be noticed by the operator. Additionally, the air conditionercompressor displacement command ramp down rate may be increased duringconditions where the energy conversion device can react faster tocounteract the additional air conditioner compressor torque. Forexample, as mentioned above, an air conditioner compressor displacementcommand may be ramped down faster so as to decrease air conditionercompressor pressure capacity when an engine is operated at speeds aboveidle speed since higher engine speed provides additional combustionevents, thereby reducing the time it takes between and engine controladjustment and torque to counteract the air conditioner compressortorque.

A different air conditioner compressor displacement ramping downstrategy may be provided when the air conditioner compressor is coupledto an electric motor. For example, the air conditioner compressordisplacement command ramp down rate may be increased at a first ratewhen the motor speed is less than base speed (e.g., motor speed wherefull motor torque is available and above which less than full motortorque is available). However, if the electric motor is at a speedgreater than base speed, the air conditioner compressor displacementcommand ramp down rate may be reduced as compared to the first rate toaccount for less available motor torque at higher motor speeds.

In addition, the air conditioner compressor displacement ramp down ratemay be adjusted depending on air conditioner system operatingconditions. For example, the ramp down rate may be increased when thedifference between an initial evaporator temperature and a final ordesired evaporator temperature is less than a first threshold (e.g., 10°C.). The ramp rate may be decreased when the difference between theinitial evaporator temperature and the desired evaporator temperature isgreater than a second threshold (e.g., 15° C.).

At time T₃, the desired air conditioner compressor clutch state remainsin a state to disengage the air conditioner compressor clutch. Further,the air conditioner compressor displacement control signal hastransitioned to a lower level so that when the air conditionercompressor clutch is disengaged, a low level torque is decoupled fromthe energy conversion device. The air conditioner compressor clutchcommand state also transitions to a lower level at time T₃ indicatingthat the air conditioning clutch is commanded to a disengaged statewhere rotational energy from the energy conversion device is nottransferred to the air conditioner compressor. Thus, during airconditioner compressor clutch disengagement, only a small load isuncoupled from the energy conversion device.

The time between T₂ and T₃ allows the torque transfer amount from theenergy conversion device to the air conditioner compressor to stabilizebefore the air conditioner compressor is decoupled from the energyconversion device. The amount of time between T₂ and T₃ can be adjustedfor air conditioner system operating conditions, vehicle operatingconditions, and energy conversion device conditions. For example, thetime between T₂ and T₃ can be increased if the energy conversion devicehas been operating for less than a predetermined amount of time. In analternative example, the time between T₂ and T₃ can be increased whenthe energy conversion device is an engine operating with fewer than itstotal number of cylinders. In particular, when the engine is operatingwith deactivated cylinders the time between T₂ and T₃ can be increasedas compared to when the engine is operating with a greater number or allengine cylinders.

In this way, the possibility of large reductions in torque between theenergy conversion device and the air conditioner compressor may bereduced. As a result, deactivation of the air conditioner systemactivation may be less noticeable to a driver.

Referring now to FIG. 7, a simulated plot illustrating air conditioningsystem control temperature versus air conditioning system torque isshown. The Y axis represents evaporator temperature. The X axisrepresents allowed air conditioning system torque consumption (e.g.,torque consumed from an energy conversion device when an air conditionercompressor clutch is engaged and an air conditioner compressordisplacement command is provided to an air conditioner compressor).

Marker 708 represents an amount of torque an air conditioner compressorapplies to an energy conversion device when an air conditionercompressor clutch is not engaged. The small amount of torque is thetorque it takes to rotate the belt and the clutch hub. Marker 710represents an amount of torque an air conditioner compressor applies tothe energy conversion device when the air conditioner compressor clutchis engaged and when the air conditioner compressor displacement commandis at a minimum level. The amount of torque that is applied to theenergy conversion device increases since there is additional mass thatrotates when the air conditioner compressor clutch is engaged and sincethe air conditioner compressor has some pumping capacity, albeit smallwhen the air conditioner compressor displacement command is at a minimumlevel.

Marker 714 represents freezing temperature of water. Marker 712represents an ambient temperature. Of course, ambient temperature canvary such that the shape of temperature curve 702 flattens out asambient temperature decreases. Further, curve 702 may be steeper in thecenter section when ambient temperature is increased. Finally, ambienttemperature can affect the lowest temperature that may be reached whenthe air conditioner compressor is operated at full capacity.

Curve 702 represents evaporator temperature and evaporator temperatureis reduced as the torque the air conditioner compressor applies to theenergy conversion device increases. Curve 702 reaches a minimum valuewhen air conditioner compressor torque is at its highest level. Thus, itcan be observed that the air conditioner compressor may provide a smallload to an energy conversion device when the air conditioner compressordisplacement command is at a lower level. Alternatively, the airconditioner compressor may provide an increased load to the energyconversion device when the air conditioner compressor displacementcommand is at a higher level. Therefore, it may be desirable to firstapply the air conditioner compressor load to the energy conversiondevice with the air conditioner compressor displacement command at alower level so that a change in torque is less perceivable to anoperator.

Vertical marker 704 represents an amount of air conditioner torque thatis permissible while powertrain torque requirements (e.g., driver demandtorque and engine accessory torque) are met by the energy conversiondevice. For example, an energy conversion device may have 400 N-M ofbrake torque available at its crankshaft at a particular engine speedand an air conditioner compressor may apply 30 N-M of torque to theenergy conversion device when the air conditioner compressor clutch isengaged and when the air conditioner compressor displacement command isat a maximum or a higher level. If the driver demand torque (e.g.,driveline torque output to vehicle wheels as requested by a driver viaan accelerator pedal) and engine accessory torque (e.g., all accessoryloads except the air conditioner including but not limited to alternatortorque, power steering torque, and vacuum pump torque) is 380 N-M, then20 N-M of air conditioner torque is permissible while still meetingdriver demand torque and engine accessory torque. Thus, the airconditioner compressor may be operated at 67% of capacity while stillproviding driver demand torque and engine accessory torque at fulllevel. The area to the left of vertical marker 704 is region wherepowertrain torque requirements may be met, but where less than full airconditioner compressor and air conditioning cooling capacity isavailable. The area to the right of vertical marker 704 is a regionwhere powertrain torque requirements will not be met if the airconditioner compressor is operated at a higher level of torque.

Horizontal marker 706 represents an evaporator temperature that may beachieved when the air conditioner compressor torque is at a level thatprovides for powertrain torque requirements. The area above marker 706represents a range of evaporator temperature that is available when theair conditioner compressor torque is at a level where powertrain torquerequirements can be met with the air conditioner operating at or belowthe air conditioner torque for meeting powertrain requirements. The areabelow marker 706 is a range of evaporator temperature that is notavailable when the air conditioner compressor torque is at a level wherepowertrain torque requirements can be met.

Thus, the plot of FIG. 7 shows that air conditioner cooling capacity canbe adjusted to meet powertrain requirement. In one example as describedby FIG. 10, full air conditioner cooling capacity is available untilpowertrain torque requirements meet a threshold level. If the powertraintorque requirements exceed the threshold level, the air conditionercompressor displacement command is adjusted such that air conditionercompressor torque decreases proportionally with the powertrain torquerequirement that exceeds the threshold level.

Referring now to FIG. 8A, a bar graph that illustrates a condition wherepowertrain torque requirements and air conditioner compressor torqueexceed the amount of available energy conversion device brake torque(e.g., engine brake torque) is shown. Specifically, bar 802 representsan amount of available energy conversion device brake torque available.Bar 804 represents an amount of powertrain requested torque, and bar 806represents an amount of air conditioner compressor torque applied to theenergy conversion device.

It can be understood from FIG. 8A that all torque demands on the energyconversion device exceed the torque output capacity of the energyconversion device. Therefore, less torque than is desired is availableto one or more of the torque consumers of the energy conversion device.As a result, the vehicle operator may notice that less torque isavailable to the vehicle driveline to propel the vehicle. Consequently,the vehicle operator may be disappointed with the vehicle's performance.

Referring now to FIG. 8B, a bar graph that illustrates a condition wherethe air conditioner clutch is disengaged so that powertrain torquerequirements may be met when air conditioner compressor torque andpowertrain torque requirements exceed the amount of available energyconversion device brake torque is shown. In particular, bar 802represents an amount of energy conversion device brake torque that isavailable. Bar 804 represents an amount of powertrain requested torque.No bar is provided for air conditioner compressor torque since the airconditioner torque is decoupled from the energy conversion device.Although the desired powertrain torque requirements are satisfied, theoperator may experience discomfort or dissatisfaction that the airconditioning system cooling capacity is deactivated.

Referring now to FIG. 8C, a bar graph that illustrates the method ofFIG. 10 is shown. Specifically, the amount of torque consumed by the airconditioning system is reduced or adjusted such that the powertrainrequested torque and the air conditioner compressor torque may beprovided by the energy conversion device.

Bar 802 represents the amount of available energy conversion devicebrake torque available. Bar 804 represents an amount of powertrainrequested torque, and bar 806 represents an amount of air conditionercompressor torque applied to the energy conversion device.

It can be observed that the air conditioner torque is reduced ascompared to in FIG. 8A so that the sum of powertrain requested torqueand air conditioner torque match the amount of torque available from theenergy conversion device without reducing the amount of powertraintorque. In this way, the driver demand torque may be provided toaccelerate the vehicle while cooling capacity remains with the airconditioning system. Alternatively, the air conditioner torque can beadjusted so that a proportion of air conditioner torque is reduced sothat powertrain torque and air conditioner torque are reduced at adesirable level.

Referring now to FIG. 9, a method for controlling an air conditioningsystem is shown. The method of FIG. 9 may be provided via instructionsexecuted by controller 12 of FIGS. 1 and 2 in the system as described inFIGS. 1 and 2.

At 902, method 900 determines operating conditions. Operating conditionsmay include air conditioning system operating conditions, energyconversion device operating conditions, and vehicle operatingconditions. Operating conditions include but are not limited toevaporator temperature, solar load, cabin humidity, cabin temperature,engine speed, engine load, motor current, and motor speed. Method 900proceeds to 904 after operating conditions are determined.

At 904, method 900 selects air conditioner compressor clutch state. Inone example, method 900 selects air conditioner compressor clutch stateas described at 340-344 of FIG. 3. In particular, method 900 receivesoperator and air conditioning system inputs. The inputs are processedvia logic and desired air conditioner compressor clutch state isselected. Method 900 proceeds to 906 after desired air conditionercompressor clutch state is selected.

At 906, method 900 determines desired evaporator temperature asdescribed at 302 of FIG. 3. In one example, the desired evaporatortemperature is continuously supplied to the controller that operates theenergy conversion device and the air conditioner compressor. If the airconditioning system is deactivated, the desired evaporator temperaturemay be set to ambient temperature. If the air conditioning system isactive, the desired evaporator temperature may be allowed to vary aboutan air conditioner evaporator control temperature as illustrated in FIG.4. Method 900 proceeds to 908 after desired evaporator temperature isdetermined.

At 908, method 900 determined a feed forward air conditioner compressordisplacement command. The feed forward air conditioner compressordisplacement command may be determined as described at 350 of FIG. 3.For example, the desired evaporator temperature may index a function ortable that outputs a variable value of a duty cycle, command voltage,command current, scaler, that provides the desired evaporatortemperature during nominal operating conditions when the variable isoutput to the air conditioner compressor displacement actuator (e.g.,control valve 20 of FIG. 1). Method 900 proceeds to 910 after the feedforward air conditioner compressor displacement command is determined.

At 910, method 900 determines expected evaporator temperature. In oneexample, expected evaporator temperature is determined as described withregard to 304 of FIG. 3 and FIG. 4. Specifically, a delay and filtertime constant are applied to the desired evaporator temperature. Inother examples, a delay and a predetermined rate limit of temperaturerise or a predetermined rate limit of temperature decay may be appliedto the desired evaporator temperature. Method 900 proceeds to 912 afterthe expected evaporator temperature is determined.

At 912, method 900 determines an evaporator temperature error. Theevaporator temperature error may be determined by subtracting the actualevaporator temperature from the expected evaporator temperature from910. Method 900 proceeds to 914 after the evaporator temperature erroris determined.

At 914, method 900 judges whether or not the evaporator temperaturethreshold is greater than a predetermined threshold. If so, method 900proceeds to 918. Otherwise, method 900 proceeds to 916.

At 918, method determined air conditioner compressor displacementcommand adjustments (e.g., adjustments to air conditioner piston stroke)from proportional, integral, and derivative (PID) terms. In one example,the air conditioner compressor command adjustments from PID terms aredetermined as described at 312-316, 320, and 324. In particular, theevaporator error is modified via proportional, integral, and derivativeterms. The integral term is limited to a predetermined level so that thecontroller doesn't continue to adjust due to a value in the integralterm when the evaporator temperature error is near zero. The PID airconditioner compressor displacement command adjustments are addedtogether and method 900 proceeds to 920.

At 916, method 900 determines air conditioner compressor displacementadjustments via a high gain memory less operation. In one example,method 900 determines air conditioner compressor adjustments accordingto 318-322 of FIG. 3. For example, the evaporator temperature error maybe multiplied by a parabolic function that increases the air conditionercompressor displacement adjustment exponentially as the evaporatortemperature error increases. Method 900 proceeds to 920.

At 920, method 900 determines the air conditioner compressordisplacement command by summing the PID, high gain memory less, and feedforward air conditioner compressor displacement commands. Method 900proceeds to 922 after the adjusted air conditioner compressordisplacement command is determined.

At 922, method 900 provides soft start and stop start for the airconditioner compressor when the air conditioner compressor is stopped orstarted. In one example, method 900 provides soft start and soft startto the air conditioner compressor as described at 328 of FIG. 3, FIGS.5-6, and FIG. 11. The soft start/stop allows a smooth torque transitionwhen the air conditioner compressor clutch is activated and deactivated.Method 900 proceeds to 924 after air conditioner clutch commands and airconditioner compressor displacement signals are adjusted to provide softair conditioner compressor stopping and starting.

At 924, method 900 provides for limiting and/or adjusting airconditioner compressor displacement command to soft start and stop startfor the air conditioner compressor when the air conditioner compressoris stopped or started. In one example, method 900 provides soft startand soft start to the air conditioner compressor as described at 328 ofFIG. 3, FIGS. 5-6, and FIG. 11. The soft start/stop allows a smoothtorque transition when the air conditioner compressor clutch isactivated and deactivated. Method 900 proceeds to 924 after airconditioner clutch commands and air conditioner compressor displacementsignals are adjusted to provide soft air conditioner compressor stoppingand starting.

At 924, method 900 limits and/or adjusts the air conditioner compressordisplacement command to control air conditioner compressor torque to adesirable level given energy conversion device available brake torqueand powertrain torque requirements. In one example, the air conditionercompressor displacement command is adjusted and/or limited as describedat 330 of FIG. 3, FIG. 8C, and FIG. 10. Specifically, the airconditioner compressor displacement command can be adjusted so that thepowertrain torque requirements are met. If the powertrain torquerequirements cannot be met via reducing the air conditioner compressordisplacement command, the air conditioner clutch may be opened touncouple the air conditioner compressor from the energy conversiondevice. The air conditioner clutch may be deactivated after the airconditioner displacement command is set to a minimum level. Method 900proceeds to 926 after the air conditioner compressor displacementcommand is adjusted based on powertrain torque requirements and energyconversion device available brake torque.

At 926, method 900 outputs the air conditioner compressor displacementcommand and the air conditioner compressor clutch command. The commandsmay be output via a duty cycle, controller area network (CAN), data bus,analog channel, or other known output type. Method 900 proceeds to 928after the air conditioner commands are output.

At 928, method 900 adjusts torque output from the energy conversiondevice to compensate for changes in the powertrain torque requirementstorque and the air conditioner compressor torque. In one example, wherethe energy conversion device is an engine, engine output can beincreased via opening an engine throttle further and increasing anamount of fuel injected to the engine. Engine spark timing may also beadjusted to adjust engine torque. Conversely, if the powertrain torquerequirements and/or the air conditioner compressor torque requirementsare reduced, the engine torque can be decreased via closing the enginethrottle and reducing the amount of fuel injected. The engine torquedemand can be adjusted according to the sum of the powertrain torquerequirements and the air conditioner compressor torque up to the WOTengine torque limit.

In another example where the energy conversion device is an electricmotor, the motor torque can be adjusted via adjusting current applied toa field of the motor. For example, if additional motor torque isrequested field current may be increased. On the other hand, fieldcurrent can be decreased to decrease motor output torque.

Thus, the method of FIG. 9 incorporates the controller as described inFIG. 3 to adjust the air conditioner clutch and air conditionercompressor of FIGS. 1 and 2. Further, the method of FIG. 9 coordinatesclutch commands and air conditioner compressor displacement commands asillustrated in FIGS. 5 and 6.

Referring now to FIG. 10, a method for limiting and/or adjusting an airconditioner displacement demand is shown. The method of FIG. 10 may beprovided via instructions executed by controller 12 of FIGS. 1 and 2 inthe system as described in FIGS. 1 and 2.

At 1002, method 900 determines available energy conversion device braketorque. In one example where the energy conversion device is an engine,available engine brake torque may be determined empirically by operatingthe engine at selected engine speeds at wide open throttle (WOT) or fullload. WOT engine brake torque for the selected engine speeds may be heldin a table or function that is stored in memory. The table or functionmay be indexed using the present engine speed and the table or functionoutputs the WOT engine brake torque which is interpreted as theavailable engine brake torque.

If the energy conversion device is a motor, the available motor torquemay be empirically determined and stored in a table or function based onthe present motor speed and field strength or current available (e.g.,maximum field current) at the present motor speed. The available braketorque may be determined via indexing the table or function using thepresent motor speed. Method 1000 proceeds to 1004 after the energyconversion device brake torque is determined.

At 1004, method 1000 determined powertrain torque requirements.Powertrain torque requirements may include operator requested drivelinetorque and engine accessory torque other than air conditioner torque.The operator requested driveline torque may be determined via reading asensor coupled to an accelerator pedal or via another type of operatorinput. In some examples, the requested driveline torque may be providedby a hybrid controller or another controller. Torque of engineaccessories may be determined via models of the accessories or fromempirically determined look-up tables that are indexed according tooperating conditions. For example, a power steering load may bedetermined based on steering wheel angle, vehicle speed, and energyconversion device speed. Alternator load may be determined based onalternator field current and alternator speed. The powertrain torquerequirement is the sum of the operator requested driveline torque andengine accessory torque. Method 1000 proceeds to 1006 after powertraintorque is determined.

At 1006, method 1000 determines an available amount of air conditionercompressor torque. In one example, available air conditioner compressortorque may be determined by subtracting the powertrain torquerequirements from the available energy conversion device brake torque.The remainder of torque may be made available to the air conditionercompressor. For example, if the available energy conversion device braketorque is 400 N-M and the powertrain torque requirements are 380 N-M,then 20 N-M of torque is available to the air conditioner compressor.

The air conditioner compressor torque may be estimated via a model thatsums air conditioner compressor friction torque, air conditionercompressor inertia torque, air conditioner pumping torque (e.g., acompressor torque based on air conditioner compressor head pressure, airconditioner compressor clutch speed and air conditioner compressordisplacement), and air conditioner dynamic pumping torque (e.g., achange in air conditioner head pressure). The model may also be invertedto determine the adjustment to torque output of the energy conversiondevice.

In other examples, more sophisticated ways to determine available airconditioner torque may be provided. For example, powertrain torquerequirements may be added to the amount torque to operate an airconditioner compressor at a desired cooling capacity to determine aconsumer torque (e.g., an amount of energy conversion device torquedemanded via the operator, air conditioner, power steering, alternator,etc.). The consumer torque is then subtracted from the energy conversionbrake torque. If the remainder is positive, the air conditionercompressor may be operated at the desired cooling capacity (e.g., theair conditioner compressor may be operated with a stroke where airconditioner compressor output pressure capacity meets the desiredcooling capacity). The amount of air conditioner compressor torque tooperate at a desired cooling capacity may be empirically determined andstored in memory for subsequent retrieval. On the other hand, if theremainder is negative, the remainder may be multiplied by a constant orfunction to determine an amount of torque reduction in the airconditioner compressor torque applied to the energy conversion device.Thus, the air conditioner torque may be reduced to exchange airconditioning cooling capacity for driveline torque or torque foraccessories other than the air conditioner.

In one example, where the available energy conversion device braketorque is 390 N-M, powertrain torque requirements torque is 370 N-M, andtorque to operate the air conditioner at full capacity is 40 N-M, theenergy conversion device brake torque capacity is 20 N-M less than theconsumer torque (390 N-M−(370 N-M+40 N-M)=−20 N-M). Therefore, at least20 N-M of torque has to be removed from the air conditioner torqueand/or the powertrain torque requirement in order to keep consumertorque below available energy conversion device torque. However in someexamples, an additional amount of torque may be removed. For example,105% of the difference between the available energy conversion devicebrake torque and consumer torque may be removed from the air conditionerand/or powertrain requirements torque to provide an excess torquebuffer. In this example, the air conditioner compressor torque may bereduced by 10 N-M so that powertrain torque can be reduced by 10 N-M.Thus, the energy conversion device torque requested by the airconditioner compressor and the powertrain torque requirements is lessthan or equal to the available energy conversion device brake torque. Inthis way, the air conditioner requested torque is reduced by an amountthat accounts for 50% of the amount of torque requested that exceeds theavailable energy conversion device brake torque to provide the availableair conditioner compressor torque of 30 N-M. The powertrain requestedtorque is reduced by an amount that accounts for 50% of the amount oftorque requested that exceeds the available energy conversion devicebrake torque, but the torque reduction is only 3% of powertrainrequirements torque. Of course, other percentages of torque reductionmay be provide via changing functions or constants that allocate torquebetween the powertrain requested torque and the air conditioner torque.

In other examples, where the air conditioner compressor requested torqueis less than is used for full air conditioner compressor coolingcapacity at a time when the powertrain torque and the air conditioningtorque exceeds the available energy conversion device torque, the airconditioner compressor cooling capacity at the present time may bereduced according to a constant or function. For example, where theavailable energy conversion device brake torque is 390 N-M, powertraintorque requirements torque is 380 N-M, and torque to operate the airconditioner at 80% of full capacity is 35 N-M, the energy conversiondevice brake torque capacity is 25 N-M less than the consumer torque(390 N-M−(380 N-M+35 N-M)=25 N-M). Therefore, 25 N-M of torque has to beremoved from the air conditioner torque and/or powertrain torquerequirement. The air conditioner compressor torque may be reduced by 15N-M so that powertrain torque is reduced by only 10 N-M to reduce therequested consumer torque to 390 N-M. Thus, the air conditionerrequested torque is reduced by an amount that accounts for 60% of theamount of consumer torque requested that exceeds the available energyconversion device brake torque to provide available air conditionercompressor torque of 20 N-M. The powertrain requested torque is reducedby an amount that accounts for 40% of the amount of consumer torquerequested that exceeds the available energy conversion device braketorque, or just about 3% of the powertrain torque requirements. In thisexample, the air conditioner compressor cooling capacity at the presenttime is reduced according to a constant or function calling for 60%reduction in air conditioner compressor torque.

The constant or function that adjusts the requested air conditionercompressor torque to provide the available air conditioner compressortorque may be indexed based on the rate of change of the desiredoperator requested driveline torque. For example, if the desiredoperator requested driveline torque changes at more than a predeterminedrate, 80% or more of the reduction in consumed torque may be a reductionin available air conditioner compressor torque. However, of the desiredoperator requested driveline torque changes at less than thepredetermined rate, less than 80% of the reduction in consumed torquemay be a reduction in available air conditioner compressor torque. Thus,if the rate of change in desired operator requested driveline torque isless than a first amount, the amount of reduction in the available airconditioner compressor torque is reduced by a first amount. If the rateof change in desired operator requested driveline torque is greater thanthe first amount, the amount of reduction in the available airconditioner compressor torque is reduced to a second amount, the secondamount greater than the first amount. In this way, the reduction inavailable air conditioner compressor torque may be based on the rate ofchange in the desired operator requested driveline torque. Method 1000proceeds to 1008 after the available air conditioner compressor torqueis determined.

At 1008, method 1000 limits the air conditioner displacement command. Inone example, the air conditioner displacement command is limited to theavailable air conditioner compressor torque. For example, if theavailable air conditioner compressor torque is 20 N-M, the airconditioner displacement command is limited to a value that provides 20N-M or less of a load on the energy conversion device via the airconditioner compressor. Thus, the combined available air conditionercompressor torque and the requested powertrain requirements torque isless than or equal to the available energy conversion brake torque. Inthis way, the available air conditioner compressor torque can be reducedor increased as requested powertrain torque varies. Method 1000 proceedsto exit after the air conditioner displacement command is limited.

Referring now to FIG. 11, a method for providing soft starting andstopping of a vehicle air conditioner compressor is shown. The method ofFIG. 11 may be provided via instructions executed by controller 12 ofFIGS. 1 and 2 in the system as described in FIGS. 1 and 2. Method 1100may provide the example sequences shown in FIGS. 5 and 6.

At 1102, method 1100 judges whether or not there is a request for airconditioner compressor state change. A request for air conditionercompressor clutch state change may be made in response in to anoperator's request to activate or deactivate an air conditioning system.For example, an air conditioner clutch state may be changed from an openstate to a closed state when additional vehicle cabin cooling isrequested. Further, a request for a change in air conditioner clutchstate may be initiated in response to an increase or decrease in vehiclecabin temperature. If method 1100 judges a request to change state ofthe air conditioner compressor clutch, method 1100 proceeds to 1104.Otherwise, method 1100 proceeds to exit.

At 1104, method 1100 judges whether or not the request to change airconditioner compressor clutch state is a request to close the airconditioner compressor clutch. If so, method 1100 proceeds to 1106.Otherwise, method 1100 proceeds to 1112. Thus, method 1100 provides twodifferent sequences for closing (e.g., starting the air conditioningsystem) and opening (e.g., stopping the air conditioning system) the airconditioner compressor clutch.

At 1106, method 1100 closes the air conditioner compressor clutch. Inone example, the air conditioner clutch may be closed via directing acurrent or voltage to the air conditioner clutch so that the airconditioner clutch is electromechanically closed. In other examples, theair conditioner clutch may be hydraulically closed. The air conditionercompressor displacement command is also reduced to a low level (e.g.,minimum level) before 1106 if it is not initially at a low level. Method1100 proceeds to 1108 after the air condition compressor clutch isclosed.

At 1108, method 1100 delays a predetermined amount of time before makingfurther air conditioner compressor adjustments. For example, the delaymay be a constant or variable depending on operating conditions asdescribed with regard to FIG. 5. Method 1100 proceeds to 1110 after thedelay time has expired.

At 1110, method 1100 ramps up the air conditioner compressordisplacement command. The air conditioner compressor displacementcommand increases the air conditioner compressor's capacity ofpressurize refrigerant and cool the evaporator when it is ramped up. Inone example, ramping up the air conditioner compressor displacementcommand increases the stroke of the air conditioner compressor piston.The ramping of air conditioner compressor displacement may be at aconstant rate as shown in FIG. 5 or dependent on a predeterminedfunction (e.g., a parabolic ramp rate). The air conditioner compressordisplacement command completes the ramp when the air conditionercompressor displacement command reaches a level equivalent to the sum ofthe feed forward gain, the PID output, and the high gain as describedwith regard to FIGS. 3 and 920 of FIG. 9. Method 1100 proceeds to exitafter the air conditioner compressor displacement command is finishedramping.

At 1112, method 1100 ramps down the air conditioner compressordisplacement command. The air conditioner compressor displacementcommand decreases the air conditioner compressor's capacity ofpressurize refrigerant and cool the evaporator when it is ramped down.In one example, ramping down the air conditioner compressor displacementcommand decreases the stroke of the air conditioner compressor piston.The ramping of air conditioner compressor displacement may be at aconstant rate as shown in FIG. 6 or dependent on a predeterminedfunction (e.g., a parabolic ramp rate). The air conditioner compressordisplacement command completes the ramp when the air conditionercompressor displacement command reaches a low or minimum level. Method1100 proceeds to 1114 after the air conditioner compressor displacementcommand is finished ramping down.

At 1114, method 1100 delays a predetermined amount of time beforeopening the air conditioner compressor clutch. For example, the delaymay be a constant or variable depending on operating conditions asdescribed with regard to FIG. 6. Method 1100 proceeds to 1116 after thedelay time has expired.

At 1116, method 1100 opens the air conditioner compressor clutch. In oneexample, the air conditioner clutch may be opened via stopping currentor voltage supplied to the air conditioner clutch so that the airconditioner clutch is electromechanically opened. In other examples, theair conditioner clutch may be hydraulically opened. Method 1100 proceedsto exit after the air conditioner compressor clutch is opened.

In this way, a torque load of an air conditioner compressor is reducedbefore the air conditioner compressor is coupled or decoupled from anenergy conversion device. Further, the progression of applying orremoving torque to the energy conversion device from the airconditioning compressor is adjusted to account for operating conditionsof the energy conversion device so as to reduce the possibility ofdisturbing the vehicle driver. Further still, operation of the energyconversion device is allowed to stabilize at a condition where airconditioning compressor load is low before the compressor is decoupledfrom the energy conversion device or before additional torque is appliedto the energy conversion device via the air conditioner compressor.

As will be appreciated by one of ordinary skill in the art, routinesdescribed in FIGS. 3 and 9-11 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

The invention claimed is:
 1. A system for controlling an air conditionerof a vehicle, comprising: an air conditioner compressor including apiston and a variable displacement control valve for adjusting a strokeof the piston; a clutch mechanically coupled to the air conditionercompressor; an electric motor supplying rotational energy to the airconditioner compressor; and a controller including instructions,executed by the controller, for adjusting the stroke of the piston inresponse to an evaporator temperature error, the evaporator temperatureerror being a difference between an expected evaporator temperature anda measured or inferred evaporator temperature, the expected evaporatortemperature determined from a desired evaporator temperature received atthe controller, and for determining air conditioner compressor torqueapplied to the electric motor and a torque adjustment to the electricmotor in response to the torque of the air conditioner compressor,wherein the stroke adjustment is limited based on available airconditioner compressor torque, wherein the expected evaporatortemperature is determined according to an engagement state of an airconditioner compressor clutch and filtering of the desired evaporatortemperature.
 2. The system of claim 1, wherein the controller furtherincludes instructions for determining available air conditionercompressor torque by subtracting powertrain torque requirements fromavailable electric motor brake torque.
 3. The system of claim 2, whereinthe controller further includes instructions for estimating an airconditioner compressor torque by summing two or more of conditionercompressor friction torque, air conditioner compressor inertia torque,air conditioner pumping torque, and air conditioner dynamic pumpingtorque.
 4. The system of claim 1, wherein the adjusting the strokeincludes, responsive to air conditioner compressor requested torquebeing less than full air conditioner compressor cooling capacity at atime when powertrain torque and air conditioning torque exceedsavailable electric motor torque, reducing the air conditioner compressorcooling capacity at the present time according to a function that isbased on a rate of change of desired operator requested drivelinetorque.
 5. The system of claim 1, wherein the controller furtherincludes instructions for determining available air conditionercompressor torque based on a rate of change in desired operatorrequested driveline torque.
 6. The system of claim 5, wherein thecontroller further includes instruction to determine if the rate ofchange in desired operator requested driveline torque is less than afirst amount, and responsive thereto reduce an amount of reduction inthe available air conditioner compressor torque by a first amount, andfurther to determine if the rate of change in desired operator requesteddriveline torque is greater than the first amount, reduce the amount ofreduction in the available air conditioner compressor torque to a secondamount, the second amount greater than the first amount.
 7. The systemof claim 1, where the expected evaporator temperature is based on anexpected rate of drop in an evaporator temperature when the airconditioner compressor clutch is closed.
 8. The system of claim 1, wherethe expected evaporator temperature is based on an expected rate of risein an evaporator temperature when the air conditioner compressor clutchis opened.